This invention relates to a process to controllably optimize the content of coal particles in a liquid medium forming a coal-water mixture. More particularly, the present invention relates to a process for producing a coal-water mixture from feedstock formed of ground, freshly-mined coal or coal salvaged from silt ponds or other source after processing to remove clay, shale, pyrite and other mineral wherein the feedstock is first classified to form two or more fractions of coal particles in a liquid medium, thereafter concentration of the coal particles in the liquid medium is determined and used for controlling the mixing together of portions of each feed stream to form a coal-water mixture having at least 65% by weight coal particles.
In my copending application Ser. No. 489,586, filed Apr. 28, 1983, there is disclosed a process for removing sulfur and ash from ultrafine coal using a feedstock which may be freshly-mined coal or coal salvaged from silt ponds or other sources. It is suitable, according to the present invention, to use the product from this process to form a coal-water mixture. One characteristic of the coal recovered from silt ponds is the substantial variation and distribution in a flow stream of the sizes of the coal particles on a day-to-day basis and possibly from hour-to-hour of operation of the process after removal of sulfur and ash from the ultrafine coal. In a similar way, a substantial variation to the particle size distribution of ultrafine sizes of freshly-mined coal can be expected when preparing feedstock for a process to form a coal-water mixture. The problem of variations to the particle size distribution of the feedstock exists in all currently-known methods for wet and dry grinding of coal.
In a paper entitled Rheology of High Solids Coal-Water Mixture by D. R. Dinger, J. E. Funk, Jr. and J. E. Funk, Sr. 4th International Symposium on Coal Slurry Combustion, May 10-12, 1982, there is described the "rheological properties of a coal-water mixture having 98.5% coal particles at 50 mesh or less depending on the particle-packing efficiency which minimizes interstitial porosity. An equation for optimum particle-packing efficiency is derived and an algorithm developed calculating the porosity of rear particle distributions. The calculated porosity was checked by pressure filtration and measurement of porosity. The specific surface area is also calculated by an algorithm. The data provides a family of particle size distributions which produce exceptional rheological properties provided the surfactant additions are effective for dispersing the coal particles. It was found that monospheres, regardless of their size will usually pack to an average orthorhombic array of about 60% by volume. In order to shear, the structure must open or dilate to a cubic array where the porosity increases from 40% to about 48%. It was found that to prevent dilatancy, or interparticle collisions to shear, the system must be diluted such that the interparticle spacing is at least IPS=(2-.sqroot.3)D, where IPS is the interparticle spacing and D is the particle size. The problem arises, however, as to the manner by which a coal-water mixture can be produced comprising at least, for example 65% by weight coal particles and preferably as high as about 82% by weight coal particles on an hourly and a day-to-day basis for reliable use. At about 65% by weight coal particles, a coal-water mixture requires the use of additional fuel such as a combustible gas when used in a power plant. However, the coat-water mixture can be economically utilized. It is, however, far more economical for usage of the coal-water mixture to increase the coal particle concentration to at least 70% by weight coal particles and up to about 82% by weight coal particles. Above 82% by weight coal particles, the delivery of the coal-water mixture by piping networks, pumps and valves will be plagued with mechanical problems.